Systems and methods are known for in-line measurement of optical coatings. In particular, various measurements such as spectral transmittance, spectral reflectance including color values, and sheet resistance measurements may be made on a coated glass sheet as it is being conveyed on a transport system in a coating apparatus.
An issue such systems face is that they must be calibrated to ensure the accuracies of their measurements.
In known systems using multiple probes provided along the width of the transport system, each probe has its own light source and its own spectrometer. Because a separate light source is used for each probe, to perform a reflection calibration, the industry standard is to reflect light off a reference material back to the spectrometer. The reference material can be a mirror or a white tile, for example. Because the probes are stationary, the reference material has to be brought in front of the probe as schematically shown in FIG. 1. The probe incorporates both a light source and a detector. The light is reflected off the reference material into the detector. This must be done for each probe that has its own light source. Because the reference material must either be brought in front of the probe or the probe must be moved to face the reference material, this complicates the operation of the measurement system and introduces issues with repeatability. In addition, over time, the reference material can become damaged or aged and thus not present a constant reflection standard and the calibration accuracy may therefore suffer. Also, because a separate light source and spectrometer is used for each measurement head, the reproducibility of the system decreases.
One such system is commercially offered by the Company of Carl Zeiss AG of Germany. It has an in-line system commercially referred to as the Optoplex system that can make various in-line measurements. Multiple pairs of probes are arranged both above and below the coating transport system. In particular, the OPTOPLEX III P provides in-line measurements of spectral reflectance and transmittance including color values. The data are available in real-time and displayed in several graphical modes. The OPTOPLEX III P says it has automatic self calibration. The calibration is believed to be performed in gaps between sheets of the coated glass. Because the light source always remains in the same position and it is not across from the reflection probe, it is believed that calibration is performed, like other prior art systems, using a reference material such as a tile as previously explained.
Other systems utilize one measurement head or probe that is mechanically translated across the width of the glass.
It is desirable to provide a system and method that overcomes these disadvantages.